Bucket-wheel trench excavators have been known to be successfully used for digging long trenches in various types of ground.
In these applications cyclic-action single-bucket excavators and continuous-action chain-bucket excavators show by far lower working rate and dependability and are incapable of giving steady performance under conditions of difficult and frozen ground.
Widely known in the art are bucket-wheel trench excavators constructed as a tracked or, occasionally, wheeled tractor carrying a mounted bucket-wheel excavating unit arranged to be raised into the transport position and be maintained therein by means of an appropriate lift mechanism, whereas in the working position the front end of said excavating unit bears on the tractor and the rear end rests on a rigid sliding support moving on the trench bottom.
For the tractor to be capable of lifting and carrying a substantially big and heavy excavating unit located with a considerable backward offset, it should possess a large margin of stability and load carrying capacity, which necessitates increase in the weight of the whole machine. It has been found that the tractor of such an excavating machine is 1.5-2 times as heavy as the excavating unit.
The extension of the scope of the bucket-wheel trench excavators, the adaptation of these machines for digging difficult and frozen ground, and also the increase in the size of piping and trenches dealt with necessitate increase in the driving power and working capacity of the machines under consideration. In consequence, such machines become heavier and less mobile, losing their capability of moving under their own power over long distances.
Owing to the disadvantages described above, mounted excavating machines cannot be used successfully, if at all, in a number of circumstances.
For heavy and powerful machines, it is more advantageous and economical to use a semitrailer arrangement which provides a wheeled support at the rear for the excavating unit to rest upon in the working and transport positions.
With such a constructional arrangement, the weight of the tractor is equal to that of the excavating unit, and the total weight of the machine becomes less. Somewhat impaired manoeuvrability of such machines does not hamper their operation as they are used mainly in conditions where much room is available.
Among such machines are advanced-design excavators described in USSR Inventor's Certificates No. 324343 Cl.E02f3/18 and No. 377486 Cl. E02f5/08.
The first-named machine has a semitrailed excavating unit connected by means of a vertical-pivoted joint to a cross which is held by horizontal-pivoted joints to a U-shaped frame mounted by means of horizontal-pivoted joints to the tractor. The transfer of the excavating unit into the transport position is effected by power cylinders which are articulated to the cross and U-shaped frame and are adapted for turning the U-shaped frame about the front and rear horizontal -pivoted joints.
With this constructional arrangement of coupling the excavating unit to the tractor, power can be transmitted to the excavating bucket wheel by hydraulic or electrical means, with resultant low efficiency and substantial increase in the weight of the machine.
A further disadvantage of this construction is a large transverse tilting moment arising in negotiating turns in the transport position and substantially impairing the transverse stability of the machine and the operating qualities thereof.
The construction of the second-named machine (Inventor's Certificate No. 377486), while retaining the advantages of the previously described arrangement, provides for employing a mechanical drive for the excavating unit. It consists of a prime mover, a bucket-wheel excavating unit arranged to bear on a wheeled support in the working and transport positions, and a system of frames composed of the prime mover frame and the excavating unit frame and connected to an intermediate frame by means of horizontal-pivoted joints. The prime mover frame has a vertical-pivoted joint which enables the excavating unit and the intermediate frame to turn in a horizontal plane relatively to the prime mover, whereby high transportation manoeuvrability is achieved and the machine is enabled to dig trenches with small radii of curvature. The transfer from the working position into the transport position and in reverse, and also the variation of the digging depth are effected by means of hydraulic cylinders articulated to the intermediate frame and to the frame of the excavating unit and arranged to turn the intermediate frame about the front and rear horizontal-pivoted joints.
The location of the front horizontal-pivoted joint near the mid-point of the track provides for even distribution of the ground pressure and high longitudinal stability of the tractor in relation to the jolting and oscillation of the excavating unit occurring in digging difficult and frozen ground, there being no substantial increase in the force exerted on the rear wheeled support.
Besides, the transfer of the vertical-pivoted joint forward from the position used in the construction described in Inventor's Certificate No. 324343 provides for good transverse stability of the machine when negotiating turns in the transport position.
Though progressive on the whole, these constructional arrangements of bucket-wheel trench excavators suffer materially from the disadvantage that great forces and loads arising in the power cylinders, articulated joints and frame system adversely affect the dependability of the machine and necessitate improving the strength and, consequently, increasing the weight thereof.
This disadvantage is attributed to the fact that in the transport position, when the intermediate frame has to be turned to the maximum extent in relation to the excavating unit frame, with the power cylinders fully extended, the axes of the power cylinders are displaced considerably toward the articulated joints which effect the connection between the intermediate frame and the excavating unit frame. These conditions give rise to particularly great forces in the power cylinders, the articulated joints and the metal structures of both frames and cause the power cylinders to make a longer stroke.
These drawbacks are particularly detrimental in the case of substantially big and heavy large-capacity excavating machines employed to dig trenches for maximum-diameter piping in difficult and frozen ground.
According to Inventor's Certificate No. 377486, the vertical-pivoted joint which enables the machine to make turns in the transport position and to dig trenches with small radii of curvature is located in the front portion of the prime mover frame, on the tractor main frame, and is constructed in the form of brackets positioned at different height and connected by two (upper and lower) shafts. The transmission which conveys drive to the excavating unit is situated midway between said brackets and inside the prime mover frame.
With respect to the freedom of turning in a horizontal plane about a vertical pivot, such a design of the connection between the tractor and the excavating unit suffers materially from the disadvantage that it has to employ complicated and cumbersome construction of the prime mover frame in order to accommodate and mount therein the bucket-wheel power transmission. Furthermore, access to the transmission as well as the servicing and installation thereof are difficult.
The construction of the vertical-pivoted joint has an important bearing on the functioning of the entire excavating machine inasmuch as this joint transmits all the large forces (such as pull, tilting effort, etc.) from the excavating unit to the tractor. Further, the construction of this joint must not complicate the operation and servicing of the machine.